Mathias,
I think what @Matthias is saying is that only the file in focus should be opened on a double-click (the other - selected - files should simply remain selected, not opened).
A real-world example would be if I wanted to move a selection of photos from one folder to another, I would select the files I know should be moved, and then open/preview the files I was not certain needed to be moved by double-clicking on them (without losing my original selection).
@Matthias, sorry if I got this wrong!